410 Part IV / Perception
Figure 18–1 The dorsal root ganglion neuron is the primary
sensory cell of the somatosensory system. The cell
body is located in a dorsal root ganglion (DRG) adjacent to the
spinal cord. The axon has two branches, one projecting to the
body, where its specialized terminal contains receptors for a
particular form of stimulus energy, and one projecting to the
spinal cord or brain stem, where the afferent signals are pro-
cessed. All DRG neurons contain five functional zones:
1. The distal terminals in skin, muscle, or viscera contain spe-
cialized receptor-channels that convert specific types of stimu-
lus energy (mechanical, thermal, or chemical) into a depolarizing
receptor potential. DRG neurons typically have multiple sensory
endings. 2. The spike generation site contains voltage-gated
Na
+
and K
+
channels (Na
V
and K
V
) that are located near the initial
segment of the axon within the receptor capsule; they convert
the receptor potential into a stream of action potentials. 3. The
peripheral nerve fiber transmits action potentials from the spike
initiation site to the DRG cell body. 4. The cell body of the DRG
neuron is contained within a ganglion adjacent to the spinal
cord or brain stem. 5. A spinal or cranial nerve connects the
DRG or trigeminal neuron to the ipsilateral spinal cord or brain
stem.
背根神经节细胞
1
2
3
4
5
外围目标(皮肤)
Damage to peripheral nerves or their targets in the
brain may produce sensory deficits in more than one
somatosensory submodality or motor deficits in spe-
cific muscle groups. Knowledge of where somatosen-
sory modalities overlap morphologically, and where
they diverge, facilitates diagnosis of neurological dis-
orders and malfunction.
Each DRG neuron can be subdivided into five func-
tional zones: the receptive zone, the spike generation
site, the peripheral nerve fiber, the DRG cell body, and
the spinal or cranial nerve (Figure 18–1). The receptive
zone, at the distal end of the DRG axon, contains spe-
cialized receptor proteins that sense mechanical force,
thermal events, or chemicals in the local environment
and translate these signals into a local depolarization
of the axonal terminals, called the receptor potential (see
Figure 3–9A). This local depolarization spreads pas-
sively toward the central axon where action potentials
are generated, usually at the initial segment (distal
to the first node of Ranvier in myelinated fibers) (see
Figure 3–10A). Stimuli of sufficient strength produce
action potentials that are transmitted along the periph-
eral nerve fiber, through the cell soma, and into the
central branch that terminates in the spinal cord or
brain stem.
The soma of a DRG neuron contains the cell
nucleus. Sensory receptor proteins are expressed in
the soma, providing a convenient expression system
for characterizing their conductance properties in
vitro. Isolated DRG neurons have been widely used in
patch-clamp studies of sensory receptor currents and
voltage-gated action potential channels.
DRG neurons differ in the size of their cell soma,
gene expression profile, conduction velocity of their
axons, sensory transduction molecule(s), innervation
pattern in the body, and physiological function. For
example, DRGs that innervate mechanoreceptors that
sense touch and proprioception have the largest cell
bodies and large myelinated axons; they express pro-
teins such as Npy2r or parvalbumin (PV) (Figure 18–2).
In contrast, DRG neurons that sense temperature or
irritant chemicals have small cell bodies and unmy-
elinated axons; they express calcitonin gene-related
peptide (CGRP) or the lectin IB4 (Figure 18–2C,D).
As these fluorescent molecular labels extend through
the axons to their peripheral endings in the body and
in the central nervous system, David Ginty and col-
leagues were able to characterize the pattern of soma-
tosensory nerve endings in the body (Figure 18–2H)
and trace their central projections to the spinal cord
(Figure 18–2G) and brain stem.
Peripheral Somatosensory Nerve Fibers
Conduct Action Potentials at Different Rates
The peripheral nerves that transmit spike trains from
the site of spike generation to the central nervous sys-
tem have classically served as the primary recording
sites for neurophysiological studies of somatosensory
receptor mechanisms. Individual peripheral nerve fibers
in animals are typically dissected from the main axon
bundle and placed on fine wires that serve as record-
ing electrodes. Microelectrodes—manufactured from
sharpened tungsten or platinum wires—have also been
inserted through the skin into the peripheral nerves of
humans (a technique known as microneurography) to
measure sensory responses to various somatic stimuli
(Chapter 19).
Peripheral nerve fibers are classified into functional
groups based on properties related to axon diameter
Kandel-Ch18_0408-0434.indd 410 19/01/21 4:05 PM
2. 脉冲生成位点包含电压门控的 Na